Voltage mode push-pull driver output stage with tunable resistances

ABSTRACT

There is provided a voltage mode push-pull driver output stage with low power consumption and improved output return loss (ORL) suitable for various high bandwidth data transmission applications. By structuring the output stage to have tunable resistances adjustable by voltages applied to transistors, the output stage is readily adaptable to different applications requiring different resistances or impedance matching.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.61/211,864, filed Apr. 3, 2009, which is hereby incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally in the field of electrical circuits.More specifically, the present invention is in the field of cabledrivers.

2. Background Art

Cable drivers are used to amplify and transmit data signals, such asvideo or audio, through interconnect cables from one device to another.For example, in professional video applications such as operating atelevision or broadcast studio, several devices with multiple input andoutput ports may need to be interconnected, including routers,distribution amplifiers, and switchers. As supporting infrastructurecontinually expands to support more devices and port interconnections,it becomes increasingly important to optimize the power consumption ofthe cable drivers in such devices to reduce operating costs and improvereliability. Furthermore, with the proliferation of high bandwidth videosuch as high definition 1080P video and the adoption of the 3G-SerialDigital Interface (SDI), it is also increasingly important to improvethe output return loss (ORL) of the cable driver to preserve the qualityof the transmitted signal.

Conventionally, current mode output stages are used in such video cabledrivers. However, current mode output stages require undesirably highlevels of power consumption and typically require large outputtransistors, which undesirably degrade ORL, especially at high bandwidth3G-SDI data transmission rates. One known method of mitigating ORLdegradation in conventional current mode cable drivers utilizes externalinductance and resistance on the outputs. However, this methodundesirably requires external components and custom tuning for eachparticular application.

Accordingly, there is a need to overcome the drawbacks and deficienciesin the art by providing a cable driver output stage with low powerconsumption and improved ORL suitable for various high bandwidth datatransmission applications.

SUMMARY OF THE INVENTION

There are provided systems and methods for voltage mode push-pull driveroutput stage, substantially as shown in and/or described in connectionwith at least one of the figures, as set forth more completely in theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will become morereadily apparent to those ordinarily skilled in the art after reviewingthe following detailed description and accompanying drawings, wherein:

FIG. 1 shows a circuit diagram of voltage mode push-pull output stage,according to one embodiment of the invention; and

FIG. 2 shows a flowchart describing the steps, according to oneembodiment of the present invention, by which a voltage mode push-pulloutput stage can operate a driver.

DETAILED DESCRIPTION OF THE INVENTION

Although the invention is described with respect to specificembodiments, the principles of the invention can obviously be appliedbeyond the specifically described embodiments of the invention describedherein. Moreover, in the description of the present invention, certaindetails have been left out to not obscure the inventive aspects of theinvention. The details left out are within the knowledge of a person ofordinary skill in the art. The drawings in the present application andtheir accompanying detailed description are directed to merely exampleembodiments of the invention. To maintain brevity, other embodiments ofthe invention which use the principles of the present invention are notspecifically described in the present application and are notspecifically illustrated by the present drawings.

Various embodiments of the present invention provide a voltage modepush-pull output stage for a driver, such as a cable driver, thatprovides substantially reduced power consumption and improved ORL, aswell as other advantages as discussed below. The invention's voltagemode push-pull output stage also provides an integrated outputtermination, such as a 75 ohm output termination.

FIG. 1 shows a circuit diagram of voltage mode push-pull output stage100 (hereinafter referred to simply as “output stage 100” in the patentapplication), according to one embodiment of the invention. Output stage100 can be an output stage for a driver, such as a cable driver and canbe driven by a pre-driver (not shown in FIG. 1). Output stage 100 has aprogrammable output impedance, which can be, for example, 75 ohms. Inone embodiment, the output impedance of output stage 100 can be 50 ohms.Output stage 100 is coupled between voltage regulator 102 and ground andincludes transistors P1 and P2, which can be, for example, P channelfield effect transistors (PFETs), transistors N1 and N2, which can be,for example, N channel FETs (NFETs), and resistors R1, R2, R3, and R4.Output stage 100 can be configured to receive input 104 and provide anoutput at output pad 106. Input 104 can be a DC voltage that can switchbetween 0.0 volts and a regulated voltage (Vreg) at a high frequency.Input 104 can have a data rate of, for example, between 100 megabits persecond (Mb/s) and 3 gigabits per second (Gb/s).

As shown in FIG. 1, input 104 is coupled to the gates of P1 and N1, thesource of P1 is coupled to voltage regulator 102, and the drain oftransistor P1 is coupled to node 110. Voltage regulator 102 can providea DC voltage of, for example, approximately 1.6 volts. In anotherembodiment, voltage regulator 102 can provide a DC voltage that ishigher or lower than 1.6 volts. Tunable resistance 114, which includesR3 coupled in series with P2 and R4 coupled across R3 and P2, is coupledbetween node 110 and node 108, which is coupled to output pad 106. Asshown, the drain of transistor P1 is coupled to node 110, tunableresistance 114 also includes transistor P1. Also shown in FIG. 1, thesource of N1 is coupled to ground and the drain of N1 is coupled to node112. Tunable resistance 116, which includes R1 coupled in series with N2and R2 coupled across R1 and N2, is coupled between node 112 and node108. As further shown, the drain of transistor N1 is coupled to node112, tunable resistance 116 also includes transistor N1.

When input 104 is at logic “1” (i.e. a logic high level), N1 conductscurrent and P1 is off, and when input 104 is at logic “0” (i.e. a logiclow level), P1 conducts current and N1 is off. When N1 is turned on,current can flow through tunable resistance 116, which can be tuned soas to provide a desired resistance between output pad 106 and ground.For example, tunable resistance 116 can be tuned to provideapproximately 75 ohms between output pad 106 and ground. Tunableresistance 116 can be tuned by applying an appropriate DC tuning voltageto the gate of N2 so as to cause a desired resistance (i.e. outputimpedance), such as approximately 75 ohms, to be provided between outputpad 106 and ground. For example, a replica circuit corresponding tooutput stage 100 can be utilized to determine the tuning voltage to beapplied to the gate of N2. In the replica circuit, N1 can be turned onwith a logic “1” and a known current can be forced into node 108,forming a loop. The voltage on the gate of N2 in the replica circuit canthen be adjusted so that the voltage at node 108 is equal to a referencevoltage, which can be determined by the resistance, such asapproximately 75 ohms, that is to be provided between output pad 106 andground. The adjusted voltage that is applied to the gate of N2 in thereplica circuit can determine the tuning voltage to be applied to thegate of N2 to achieve the desired resistance between output pad 106 andground in output stage 100.

In one embodiment, the DC tuning voltage applied to the gate of N2 canbe controlled by means of an external resistance Rset and the replica ofoutput stage 100. A loop can set the DC tuning voltage so that theequivalent replica impedance is proportional to the external resistance.

When P1 is turned on, current can flow through tunable resistance 114,which can be tuned so as to provide a desired resistance, such asapproximately 75 ohms, between voltage regulator 102 and output pad 106.Tunable resistance 114 can be tuned by applying an appropriate DC tuningvoltage to the gate of P2 so as to cause a desired resistance (i.e.output impedance), such as approximately 75 ohms, to be provided betweenvoltage regulator 102 and output pad 106. Tunable resistance 114 can betuned in a similar manner as tunable resistance 116. For example,another replica circuit corresponding to output stage 100 can beutilized to determine the tuning voltage to be applied to the gate ofP2. In the replica circuit, P1 can be fully turned on by applying alogic “0” to the gate of P1. A known current can be injected into thesource of P1 at voltage regulator 102 and the gate of P2 can be adjustedto provide a reference voltage at node 108. In one embodiment of theinvention, the reference voltage can be approximately 1.2 volts, whichcan correspond to a logic “1” at output pad 106. The adjusted voltagethat is applied to the gate of P2 in the replica circuit can determinethe tuning voltage to be applied to the gate of P2 to achieve thedesired resistance between voltage regulator 102 and output pad 106 inoutput stage 100.

In one embodiment, the DC tuning voltage applied to the gate of P2 canbe controlled by means of an external resistance Rset and the replica ofoutput stage 100. A loop can set the DC tuning voltage so that theequivalent replica impedance is proportional to the external resistance.

In output stage 100, output pad 106 can be AC coupled to a load by acapacitor, for example. Output stage 100 can provide an output pulse atoutput pad 106 having a peak-to-peak voltage swing of, for example,approximately 800 millivolts (mV)±10 percent. To achieve a voltage swingof approximately 800 mV, the logic low can be approximately 0.4 voltsand the logic high can be approximately 1.2 volts at output pad 106.

When N1 is on and P1 is off, current can flow through tunable resistance116 and N1 and a logic “0” (logic low) is produced at output pad 106.When N1 is on, an output impedance, such as a 75 ohm impedance, can bedefined between output pad 106 and ground by the bottom half of outputstage 100. When P1 is on and N1 is off, current can flow from voltageregulator 102 through P1 and tunable resistance 114 to output pad 106.The current flowing to output pad 106 can provide a voltage at outputpad 106 that defines a logic “1” (logic high). When P1 is on, an outputimpedance, such as a 75 ohm impedance, can be defined between voltageregulator 102 and output pad 106 by the top half of output stage 100.

In output stage 100, current flowing in the internal output terminationactually contributes to the voltage swing and the output is trulysingle-ended. The current required to provide amplitude Vamp when outputpad 106 is coupled to a receiver having a termination Rterm can beI=Vamp/(2·Rterm) (assuming Rterm is equal to the output termination ofoutput stage 100). In a conventional current mode driver, the internaloutput termination is in parallel with the receiver output termination.As a result, the current required to provide the same output amplitudecan be I=(2·Vamp)/Rterm. Consequently, the conventional current modedriver can require four times as much current as output stage 100 toprovide the same output voltage swing. Also, the conventional currentmode driver provides differential outputs. Thus, current is wasted inthe conventional current mode driver when both outputs are not beingutilized. Thus, the invention's voltage mode push-pull output stagesubstantially reduces power consumption compared to a conventionalcurrent mode driver.

Also, since there is less current flowing into N1 and P1, thosetransistors can be smaller and, consequently, easier to drive. As aresult, the circuit (e.g. pre-driver) driving the input of output stage100 can advantageously consume less power and can be easier to design.In output stage 100, P2, N2, R1, R2, R3, and R4 can also be reduced insize, which can reduce manufacturing cost. Also, output stage 100 canadvantageously consume less area on an IC chip.

Additionally, since N1 and P1 are smaller, they have less parasiticcapacitance. As result, the parasitic capacitance on output pad 106 canbe reduced, which can advantageously improve return loss at highfrequency.

Further, the invention's voltage mode push-pull output stage can provideimproved duty cycle dispersion (DCD) characteristics, therebyadvantageously reducing jitter.

Moreover, by utilizing a voltage regulator, the invention's voltage modepush-pull output stage can advantageously provide increased power supplynoise rejection.

Moving to FIG. 2, FIG. 2 shows a flowchart describing the steps,according to one embodiment of the present invention, by which a voltagemode push-pull output stage can operate a driver. Certain details andfeatures have been left out of flowchart 200 of FIG. 2 that are apparentto a person of ordinary skill in the art. For example, a step mayconsist of one or more sub-steps or may involve specialized equipment,as known in the art. While steps 210 through 240 shown in flowchart 200are sufficient to describe one embodiment of the present invention,other embodiments of the invention may utilize steps different fromthose shown in flowchart 200.

At step 210, the output stage adjusts a first tunable resistance betweena first node and a third node by adjusting a first tuning voltageapplied to a gate of a second transistor. Referring to FIG. 1, thiscorresponds to output stage 100 adjusting tunable resistance 114 betweennode 110 and node 108 by adjusting a first tuning voltage applied to agate of transistor P2. As described above, feeding a known currentthrough a replica circuit of output stage 100 may be utilized todetermine the appropriate first tuning voltage for setting a desiredoutput impedance between voltage regulator 102 and output pad 106, suchas 50 or 75 ohms. Alternatively, as also described above, the firsttuning voltage may be controlled by means of external resistance and thereplica circuit.

At step 220, the output stage adjusts a second tunable resistancebetween a second node and the third node by adjusting a second tuningvoltage applied to a gate of a fourth transistor. Step 220 may becarried out in a manner similar to step 210. Referring to FIG. 1, thiscorresponds to output stage 100 adjusting tunable resistance 116 betweennode 112 and node 108 by adjusting a second tuning voltage applied to agate of transistor N2. As described above, feeding a known currentthrough a replica circuit of output stage 100 may be utilized todetermine the appropriate second tuning voltage for setting a desiredoutput impedance between ground and output pad 106, such as 50 or 75ohms. Alternatively, as also described above, the second tuning voltagemay be controlled by means of external resistance and the replicacircuit.

At step 230, the output stage receives an input signal coupled to a gateof a first transistor and a gate of a third transistor. Referring toFIG. 1, this corresponds to output stage 100 receiving input 104 coupledto a gate of transistor P1 and a gate of transistor N1.

At step 240, the output stage conducts current through the secondtunable resistance when the input signal is at logic high or through thefirst tunable resistance when the input signal is at logic low. Asdiscussed above, when transistor N1 receives input 104 as logic high,current travels through tunable resistance 116, and transistor P1 goesinto an off state. As previously described, output stage 100 may beconfigured such that output pad 106 receives approximately 0.4 voltsrepresenting logic low. On the other hand, when transistor P1 receivesinput 104 as logic low, current travels through tunable resistance 114,and transistor N1 goes into an off state. As previously described,output stage 100 may be configured such that output pad 106 receivesapproximately 1.2 volts representing logic high. As a result, input 104is efficiently transmitted by voltage mode push-pull driver output stage100 through output pad 106, which may, for example, further transmit thesignal through video cable interconnects to another device.

From the above description of the invention it is manifest that varioustechniques can be used for implementing the concepts of the presentinvention without departing from its scope. Moreover, while theinvention has been described with specific reference to certainembodiments, a person of ordinary skills in the art would recognize thatchanges can be made in form and detail without departing from the spiritand the scope of the invention. As such, the described embodiments areto be considered in all respects as illustrative and not restrictive. Itshould also be understood that the invention is not limited to theparticular embodiments described herein, but is capable of manyrearrangements, modifications, and substitutions without departing fromthe scope of the invention.

What is claimed is:
 1. A method of operating a driver for use by avoltage mode push-pull output stage having a voltage regulator, themethod comprising: adjusting a first tunable resistance having a firstresistor coupled to an output; adjusting a second tunable resistancehaving a second resistor coupled to the output; receiving an inputsignal, the input signal being coupled to a gate of a first transistorand a gate of a second transistor at a single node, wherein the voltageregulator is coupled to a source of the first transistor, and wherein asecond end of the second tunable resistance is coupled to a drain of thesecond transistor, and a second end of the first tunable resistance iscoupled to a drain of the first transistor; and conducting a currentthrough the second tunable resistance when the input signal is at logichigh or through the first tunable resistance when the input signal is atlogic low.
 2. The method of claim 1, wherein the first transistor is a Pchannel field effect transistor (PFET), and wherein the secondtransistor is an N channel field effect transistor (NFET).
 3. The methodof claim 1, wherein the adjusting of the first tunable resistance is byadjusting a first tuning voltage.
 4. The method of claim 3, wherein theadjusting of the first tuning voltage is controlled by an externalresistance.
 5. The method of claim 1, wherein the adjusting of thesecond tunable resistance is by adjusting a second tuning voltage. 6.The method of claim 5, wherein the adjusting of the second tuningvoltage is controlled by an external resistance.
 7. The method of claim1, wherein the voltage regulator provides a DC voltage of approximately1.6 volts.
 8. A voltage mode push-pull output stage for a driver, theoutput stage comprising: a voltage regulator; a first, a second, and athird node, wherein the third node is coupled to an output pad; a first,a second, a third, and a fourth resistor; a first, a second, a third,and a fourth transistor, wherein a source of the first transistor iscoupled to the voltage regulator, a drain of the first transistor iscoupled to the first node, a gate of the first transistor is coupled toan input signal, a source of the third transistor is coupled to aground, a drain of the third transistor is coupled to the second node,and a gate of the third transistor is coupled to the input signal; afirst tunable resistance between the first node and the third node, thefirst tunable resistance comprising the third resistor coupled in serieswith the second transistor and the fourth resistor coupled in parallelto the third resistor and the second transistor, the first tunableresistance adjusted by a first tuning voltage applied to a gate of thesecond transistor; and a second tunable resistance between the secondnode and the third node, the second tunable resistance comprising thefirst resistor coupled in series with the fourth transistor and thesecond resistor coupled in parallel to the first resistor and the fourthtransistor, the second tunable resistance adjusted by a second tuningvoltage applied to a gate of the fourth transistor.
 9. The output stageof claim 8, wherein the first transistor and the second transistor eachcomprise a P channel field effect transistor (PFET), and wherein thethird transistor and the fourth transistor each comprise an N channelfield effect transistor (NFET).
 10. The output stage of claim 8, whereinthe second tuning voltage is adjusted such that an output impedance ofapproximately 75 ohms is provided between the output pad and the ground.11. The output stage of claim 8, wherein the second tuning voltage iscontrolled by an external resistance and a replica circuit of the outputstage, wherein a loop sets the second tuning voltage so that anequivalent replica impedance of the replica circuit is proportional tothe external resistance.
 12. The output stage of claim 8, wherein thefirst tuning voltage is adjusted such that an output impedance ofapproximately 75 ohms is provided between the output pad and the voltageregulator.
 13. The output stage of claim 8, wherein the first tuningvoltage is controlled by an external resistance and a replica circuit ofthe output stage, wherein a loop sets the first tuning voltage so thatan equivalent replica impedance of the replica circuit is proportionalto the external resistance.
 14. The output stage of claim 8, wherein theoutput pad is AC coupled to a load by a capacitor, and wherein theoutput stage is configured to provide a peak-to-peak voltage swing ofapproximately 800 millivolts (mV)±10 percent where logic low at theoutput pad is approximately 0.4 volts and logic high at the output padis approximately 1.2 volts.
 15. The output stage of claim 8, wherein thevoltage regulator provides a DC voltage of approximately 1.6 volts. 16.The output stage of claim 8, wherein the input signal comprises a DCvoltage switching at high frequency between 0.0 volts and a regulatedvoltage at a data rate between 100 megabits per second (Mb/s) and 3gigabits per second (Gb/s).
 17. The output stage of claim 8, wherein thedriver is a video cable driver.
 18. A method of operating a driver foruse by a voltage mode push-pull output stage having a voltage regulator,a first, a second, and a third node, a first, a second, a third, and afourth resistor, and a first, a second, a third, and a fourthtransistor, wherein the third node is coupled to an output pad, a sourceof the first transistor is coupled to the voltage regulator, a drain ofthe first transistor is coupled to the first node, a source of the thirdtransistor is coupled to a ground, and a drain of the third transistoris coupled to the second node, the method comprising: adjusting a firsttunable resistance between the first node and the third node byadjusting a first tuning voltage applied to a gate of the secondtransistor, the first tunable resistance comprising the third resistorcoupled in series with the second transistor and the fourth resistorcoupled in parallel to the third resistor and the second transistor;adjusting a second tunable resistance between the second node and thethird node by adjusting a second tuning voltage applied to a gate of thefourth transistor, the second tunable resistance comprising the firstresistor coupled in series with the fourth transistor and the secondresistor coupled in parallel to the first resistor and the fourthtransistor; receiving an input signal coupled to a gate of the firsttransistor and a gate of the third transistor; and conducting a currentthrough the second tunable resistance when the input signal is at logichigh or through the first tunable resistance when the input signal is atlogic low.
 19. The method of claim 18, wherein the first transistor andthe second transistor each comprise a P channel field effect transistor(PFET), and wherein the third transistor and the fourth transistor eachcomprise an N channel field effect transistor (NFET).
 20. The method ofclaim 18, wherein the adjusting of the second tuning voltage iscontrolled by an external resistance and a replica circuit of the outputstage, wherein a loop sets the second tuning voltage so that anequivalent replica impedance of the replica circuit is proportional tothe external resistance.